DENVER (AP) – The Colorado Division of Oil and Public Safety says no cause has been determined for the premature explosion of an avalanche shell that injured two state workers on Monday.

Public Safety Program Manager Greg Johnson said in a statement Wednesday that possible causes include equipment malfunction or operator error.

The explosion of a shell inside a gun used to trigger controlled landslides to protect drivers and backcountry skiers seriously injured a state transportation worker and also injured an avalanche forecaster.

The two men were standing behind a barricade in the area about 50 miles west of Denver when the shell went off, injuring the men above the waist.